Mole concept...

L = Avogadro constant

1 formula unit of MgCl2 contains 1 Mg²⁺ ion and 2Cl¯ ions.
Hence, L formula units of MgCl­2 contains L Mg²⁺ ions.

Since 1 mole of MgCl2 = L formula units of MgCl2, and 1 mole of Mg²⁺ ions = L Mg²⁺ ions,
thus 1 mole of MgCl2 contains 1 mole of Mg²⁺ ions.

2010-10-05 22:50:50 補充：
Note that MgCl2 is an ionic compound, which contains Mg²⁺ and Cl¯ ions, but contains no molecules.

可以用餐具黎了解

一套餐具(one molecule of MgCl2)
有1隻匙 (Mg 2+)
同2隻筷子(Cl2 -)

依家有1 打(mole) 餐具(MgCl2), 有幾多個匙(Mg 2+)?
答: 1打(mole)

希望你明啦
中文係比喻

2010-10-07 18:32:42 補充：
sor 錯左...

係

一套餐具(one unit of MgCl2)

to add a point, MgCl2 is NOT molecule. it's ionic compound -- molecule refers to discrete, simple molecules.

It's because 1 mole of MgCl2 has 1 mole molecule. Imagine that MgCl2 is a Lego, now, the Lego needs 1 Mg and 2 Cl to become MgCl2, now there's 100 Lego.So, there should be 100 Mg in this 100 Lego.

In real situation, 1 mole MgCl2 is just like 1 moe Lego and all the Lego contains 1 mole of Mg. In the same way, there should also be 2 mole of Cl in 1 mole of MgCl2